Head to Porirua and then north to Titahi Bay township. Find your way to Whitireia Park via Thornley St. After noting the time that the gate closes head past it for 250m and park by some old concrete foundations. There's a track that leads down a spur to the cliffs. Alternatively, head uphill to locate a second track that's useful at high tide/big seas.
This link has a comprehensive guide: http://climbnz.org.nz/nz/ni/wellington/titahi-bay